This invention relates generally to the field of data communications, and more specifically to data communications wherein the total time delay between a transmitted and received signal may vary, either because of changes in absolute distance between the transmitter and receiver or because of routing changes in the path between the transmitter and receiver.
It is commonly necessary to maintain such total time delay constant, for example, in satellite communication systems where all units are synchronized to a common station clock. Even a "synchronous" satellite, if positioned at a point above the earth not directly above the equator, will move in a figure-eight pattern with a potential maximum movement of several hundred miles. Such a deviation may cause periodic loss of synchronization, hence loss of communications, which must be continually detected and corrected. The technique previously employed to correct this problem is typified in the disclosure of U.S. Pat. No. 3,611,435 to Cooper, which was granted on Oct. 5, 1971. As described therein, a signal is transmitted from a ground terminal to a satellite and back to the ground terminal. The total round-trip delay is measured, and the transmission delay is varied to maintain constant the total time delay measured.
A primary disadvantage of this method is the requirement that the transmitted signal be retransmitted from the receiver back to the transmitter, with subsequent corrections being made at the transmitter. It would considerably reduce the time required for resynchronization if the need to retransmit the received signal back to the transmitter could be eliminated. Additionally, it would then be possible to eliminate transmitter equipment in the receiver if such equipment were otherwise unnecessary for a particular application.